WASHINGTON — Defense Secretary Ash Carter said Tuesday that the US plans to step up its attacks against Islamic State militants in Iraq and Syria, possibly escalating American action on the ground.

‘‘We won’t hold back from supporting capable partners in opportunistic attacks against ISIL, or conducting such missions directly whether by strikes from the air or direct action on the ground,’’ Carter told the Senate Armed Services Committee, using an acronym for the militant group that holds large parts of Iraq and Syria.

The US has done some special operations raids in Syria. Carter did not say under what circumstances the US might act on the ground on its own, but said ‘‘once we locate them, no target is beyond our reach.’’

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Carter’s testimony described a changing approach to the fight against the Islamic State, focusing largely on Raqqa, the militants’ declared capital in Syria, and Ramadi, the capital of Anbar province in western Iraq. It reflected an acknowledgment of little recent progress in defeating the militant group.

Carter said the US would intensify the air campaign against the Islamic State with additional US and coalition aircraft and heavier airstrikes. His testimony came as Russia is conducting its own airstrikes in Syria, saying it aims to help the Syrian government defeat the Islamic State and other terrorists.

While both the US and Russia oppose the Islamic State, Russia is supporting Syrian President Bashar Assad, whom the US wants out of power. Some Republican lawmakers complained that the Obama administration isn’t doing enough against Assad.

Carter said the US-led effort will include more strikes against the Islamic State’s ‘‘high-value targets as our intelligence improves, and also its oil enterprise, which is a critical pillar of IS’s financial infrastructure.’’

Carter said to keep up the pressure on Raqqa, the US will support moderate Syrian forces, who have made territorial gains against the Islamic State near that city. ‘‘Some of them are within 30 miles of Raqqa today,’’ he said.

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He said the US also hopes to better equip Arab forces battling the Islamic State and to further bolster Jordan, a neighbor of Iraq and Syria which is flying missions as part of the anti-IS coalition.

Carter said he was disappointed that the US effort to form new moderate Syrian rebel forces to fight IS had failed. He said the new approach is to work with vetted leaders of groups that are already fighting the militants and also give them equipment and training and help support them with US air power.

‘‘If done in concert as we intend, all these actions on the ground and from the air should help shrink IS territory into a smaller and smaller area and create new opportunities for targeting IS — ultimately denying this evil movement any safe haven in its supposed heartland,’’ Carter said.

The new strategy also includes helping the Iraqi government’s effort to assemble Iraqi forces, including Sunni fighters, to fight Islamic State militants in Anbar province. Carter said that as the US sees more progress in assembling motivated Iraqi forces, it will be willing to continue providing more equipment and fire support to help them succeed.

‘‘However, the Iraqi government and security forces will have to take certain steps militarily to make sure progress sticks,’’ he said.

Carter’s outline of the new US approach came under attack by Republicans on the committee.

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Republican Sen. Lindsey Graham of South Carolina pressed both Carter and Marine Gen. Joseph Dunford Jr., chairman of the Joint Chiefs of Staff, on whether the US has a military strategy to take out the Syrian president. Graham noted that Russian, Iranian and the Hezbollah militant group all are supporting Assad. Carter and Dunford both repeatedly said that the US was supporting moderate forces in Syria in the fight against the Islamic State, but not those fighting against Assad.

Carter said the US approach to removing Assad has been mostly a political effort.

Dunford said, ‘‘I think the balance of forces right now are in Assad’s advantage.’’

Graham seized on their replies, saying Assad is ‘‘secure as the day is long.’’

‘‘If I’m Assad this is a good day for me because the American government has just said, without saying it, that they are not going to fight to replace me,’’ Graham said.

‘‘You have turned Syria over to Russia and Iran. ... This is a sad day for America and the region will pay hell for this,’’ he said. ‘‘The Arabs are not going to accept this. The people of Syria are not going to accept this.’’